Implementing EXIT_SAPLCSLS_002 to send VBAP-ZFIELD to APO

I need some help in implementing EXIT_SAPLCSLS_002 to send VBAP-ZFIELD to APO when we execute CFM1 while generating Integration Model. This field needs to be passed to APO along with a Sales Order or Scheduling agreement.

I have gone through the fields available within the Exit. There is an extension parameter as well. But I could not find any documentation on how to implement this.